Taliban insurgents in Afghanistan today shot dead a British aid worker who they said had been spreading Christianity.

Gayle Williams, 34, was killed by two men on a motorbike as she walked to work in the capital, Kabul, at about 8am (0430 BST).

Mike Lyth, the chairman of Serve Afghanistan, the organisationWilliams worked for, said: "She was almost at the office when a couple of guys jumped off a motorcycle with some kind of weapon and shot her. She was dead almost immediately.

An undated photograph of the British aid worker Gayle Williams, who was shot dead in Kabul. Photograph: Serve Afghanistan/PA Serve Afghanistan/PA/PA

"One of our staff came on the scene fairly soon afterwards and found her dead. Now they are trying to live with the consequences of that.

"We are in deep shock about it all because we knew her very well."

Zemeri Bashary, the Afghan interior ministry spokesman, said Williams had been shot in the body and leg with a pistol.

The international development secretary, Douglas Alexander, condemned the Taliban's attempt to justify the killing as a religious act. "It was cold-blooded murder," he said.

Williams, who had dual British and South African citizenship, had worked for Serve Afghanistan for two years and was considered an experienced member of staff who did not take undue risks.

The British-based Christian aid charity focuses on community development, education and vocational training for people with disabilities. The group's 15 expatriate staff are all volunteers.

Williams had recently moved back to Kabul from Kandahar because of concerns about the security of the charity's office there.

The Taliban said it was responsible for her killing, which it had carried out because she had been promoting Christianity.

But Lyth, the Carlisle-based chairman of Serve Afghanistan, denied that Williams had been involved in missionary work. She spoke the local languages Pashtu and Dari but was fluent in neither, he said.

"They [the Taliban] will make any excuse. They probably saw there was a Christian organisation operating in Kabul and thought, 'This is how we can kill it'.

"We are Christians. That is what gives us the motivation to go into a dangerous and difficult country to try to help. But she was not involved in proselytisation."

Describing Williams as a "lovely girl", Lyth added: "She was the life and soul of the party, a great adventurer. She will be sorely missed - a great girl.

"It is a tragedy that she was killed in Kabul, which is supposed to be relatively safe."

Williams was brought up in South Africa and spent some years in Britain, where her mother lives. She has a sister in South Africa.

Lyth said the charity would now have to take a "long, hard look" at its operations.

"I personally have been very concerned about security for a long time, but we have tried to take all possible measures to reduce the threat.

"We train our people really carefully. We are in daily touch with the security authorities to find out which roads we shouldn't be on, which parts of the country we shouldn't go to.

"Each time something like this happens, you wonder: do you go on exposing people to unnecessary risk? Yet at the same time, you have got the cry of many, many of the Afghans saying, 'Please help us'. You're caught between a rock and a hard place."

Taliban insurgents have increasingly targeted aid workers this year in their campaign to undermine support for the western-backed Afghan government.

Taliban insurgents killed three female aid workers and their Afghan driver in an ambush just outside Kabul in August, in the bloodiest single attack on foreign humanitarian workers in Afghanistan in recent years.

The violence has already forced some aid agencies to restrict their activities in Afghanistan.